The gas-air mixers of gas engines, which are provided for all the cylinders in common, are conventionally designed such that the mixture ratio of air to gas, .lambda.v, remains constant over the entire load range of the engine. If a gas with a different calorific value, for instance a higher calorific value, such as natural gas instead of sewage gas, is used, the result is that the gas with the higher calorific value also has a considerably greater air requirement. If the air quantity is not varied, then the calorific value of the mixture in the air-gas charge of a cylinder is virtually independent of the calorific value of the gas. Yet that means in turn that when a gas having a higher calorific value is used, the quantity of gas must be reduced at full load by a corresponding throttling in the gas pressure regulator. An optimal .lambda.v can thereby be attained for full load. Since a throttle of this kind becomes less and less effective as the load drops and hence with a decreasing quantity of gas per unit of time, .lambda.v then drops sharply in the partial-load range, in fact increasingly as the load decreases. The consequence is combustion at very high temperatures, perhaps in the vicinity of the knocking limit, and a sharp increase in nitric oxide emissions.